Continuity: So Easy to Check, Except When It's Not

I'm still looking for a test unit which solves my one-wire continuity problem.

Checking for DC continuity is one of the most basic tests engineers can make when verifying or debugging a circuit. After all, it's important, it's easy to do, and having poor or no continuity will ruin performance of many systems.

Fortunately, in most cases continuity is also easy to check. If it's a PC board, an installed short cable, or a long cable on the bench, you can usually check it out with a meter in a few seconds. To make it easier, many DMMs have an audible continuity mode which lets you know if the resistance is below a fixed amount, such as an Ohm, so you can "buzz out" the pathway without having to look at the meter reading. Instead, you can look at your probes and where you are placing them.

But there are times when something as simple as continuity is actually hard to assess, due to the physical layout. I'm not talking about a tight PCB, or a multilayer one with internal ground and power planes, populated with vias that could have hairline cracks. I mean where the two ends of the single wire under test (I'll call it the WUT) are very far apart.

I thought about this a few weeks ago, when I caught a part of an episode of This Old House. The show mentioned a meter that lets you check continuity of a wire from one end alone, or so it seemed to me. Great, I thought, tell me more: Maybe there's some sort of transponder you put at the far end and it somehow it relays back to the source (your end) the continuity of the single wire.

The one-wire continuity problem was one I had faced in the past, when I was helping a friend fixing up a very old two-story house. We found a single red-insulated wire coming out of a defunct wall-switch box on one floor, and similarly situated wire on the other floor. Were they the ends of same wire? Did they have continuity now? Did they ever have it?

Since there were no other wires of known good continuity nearby, we had a challenge. "Continuity" means a path for electrons to flow, but they need a return path to the source. We tried using the grounding screw on the junction box at one end and a similar one at the other end for the return path connections, but the reading indicated "open circuit." We had little faith in this result, however, because ground connections in old buildings are often corroded or disconnected. So the determination problem was still there.

In the end, we did the simple, obvious thing: We took a long piece of basic #20 wire, ran it from one floor to the other through the windows and outside, and used it as the return path. That solved the problem, which let us run a valid quick test (and the WUT was OK).

I thought this Extech continuity-test unit would solve a long-standing problem, but I misunderstood its function.

Out of curiosity, I looked into the unit they cited on This Old House, the Extech CT20 Remote & Local Continuity Tester. It's certainly handy, but it doesn't let you check one-wire continuity. I must have misunderstood what they said it did. To use it, you connect your two wires at the far end (one of which must be known to be continuous), then buzz them out from the near end. It even has some diodes in the far-end jumper cable so you can even buzz out a bunch of wires in the cable from the source end, if you attach their multiple-clip jumper.

Next time, I'll just have to get out the TDR (time domain reflectometer) to check that one wire—perhaps that will do the trick. (Well, not really an available option at my job site.) As far as I can tell, there is no easy or low-cost way to verify the continuity of a single wire over a distance without a known good return-current path.

Are there any basic, low-cost test instruments you'd like to see, that would solve such "basic" and obvious problems?

I am Mike, Doaks friend and primary maintence guy. I was an electronic weapons tec in the Navy. I keep a 100ft and 50ft coil of 22ga speaker wire with my ohm meter. I just use a cheepo $19 meter. My speaker wire has clips so clip one end to that pesky wire, roll the wire back to where you want to test and clip its other end to my negative probe. Start testing! The Navy does not teach this in school but they teach you to think! In a crisis or emergency, when your life,especially the lives of others, you better figure it out! And dont take all damn day, thats what my instructor taught us. I set for tone and in 30y its never failed me! I took 3 100ft extension cords one time, shorted one end, tested the other and got tone down 300ft of one side and 300ft back. So I streched it out, hooked one side to what I needed to test and I found my wire. Also look across other fields, a friend of mine has a phone toner! This is for 2 wire test! Its made for phones. When you pull 200 wires from the phone room and run em all over you have to know which is which! You plug this little thing into a phone jack, the other piece is a non contact wand! It will tell you which wire out of 200 you are pluged to! Super sweet! Take a phone jack with 3in phone wire hook to male plug insert in any outlet. This is going to allow you to id every outlet, light and switch and the breaker! Power must be off! Its also best to pop the breakers off the panel! thats easier than pulling wires off the breakers! I myself never tried it but I watched him do it more than once! I think its $300.

Commonly called a Tic Tracer, Triplett makes a version Sniff-It 2. With a variable setting, it beeps and flashes a light near an energized circuit. Beeping becomes more frequent the nearer you get. Rated for 5V - 600VAC, it is a non contact device great for finding live wires behind dry wall.

I think your time domain reflectometer will bounce signals off of every wire nut connection.

I just use RF out on the spec analyser, but it's kind of big to lug around.

Years ago I remember there were hand made testers that generated swept signals around 455kHz or 10.7MHz or 45MHz. You could test all the way from the antenna through to the speaker / TV screen, (AM /FM didn't matter) They relied on leakage of the IF through the front end so you didn't need to tune the radio. (If you have a spec analyserswith a tracking generator you can make a similar test signal)

For locating the other end of a single wire , I just hook up a 20.00MHz crystal oscilator to 4.5v worth of battery and attach the crystal oscillator output to the cable. You can do all this with 3 jumper cables. Then just use a FM radio tuned to 100MHz to follow the wire. You can use any crystal oscillator that has an odd harmonic in the FM band.

If I need a TDR , I just connect a Tee to the scope input, and put the "1kHz probe comp" signal into one end of the tee through say a 100ohm resistor and connect the other end to the cable. Do a calibration check on a known piece of cable and you are away!.

The small 8-wire network cable testers usually use a CD4017 counter chip to put a + on one wire at a time and use any or all of the other wires for return...so you need at least 2 wires working. You can use the same technique and a bunch of 74HC154 dedoders to expand that to 16 x n lines. So you could build yourself a custom cable checker quite cheaply if you don't mind picking up a soldering iron.

Of course, I don't know if would be worth your while doing this - I'm not sure how many pipe-organ axe-murderers there are out there.....

@Bill the continuity threshold was about 5 ohms. Of course, your value may differ!

Our lab DMMs buzz at 33 ohms - useless! So I built a bunch of "milliohm squawkers" for our techs to use. This changes delta-R of a few milliohms into a variable pitch tone so a reverse-engineering tech can audibly distinguish correct PCB connections in spite of closed relay contacts or low resistance transformer windings. Watch for it in upcoming EDN Design Ideas.

Speaking of continutity testing, the other day I was frustrated because I needed to check RF continuity through a path with series AC coupling caps. DMM is going to show near open circuit, and I wondered if a device to inject an RF tone at a certain frequency (user settable of course) and detect it at the other end of the path would be a marketable product. What do you think? I'd buy one!

What about step attenuators with pre- or post-amplifier or both to compensate for non-0 dB insertion loss, bypassable for those situations where you don't want the noise or distortion from the amp(s)? And/or built-in power detector for ALC (automatic level control)? Any takers?

I'm just throwing things out there, lest you think I'm doing my own marketing in a public forum. If anyone from Mini-Circuits or Telemakus or wherever is listening, maybe this is an opportunity to make some useful instruments.

@MeasurementBlues, you make an excellent point about the audible (DC) continuity testers - what is the resistance threshold for the beep? I often say, "I don't trust the beep." and set the DMM to read the ohms, thank you very much. Now, if I could set the ohm threshold, I might be inclined to buzz my circuits out.